In order to understand the design of a safety system based on a medical model, an understanding of human anatomy and the mechanisms of injury thereto is necessary.
Referring to FIG. 1, a lateral side elevation view of human spine 10 is shown. The spine is a flexible and flexious column made up of a series of bones called vertebrae. Each person typically has 33 vertebrae. These are broken up into five types according to position. They are: cervical, dorsal ("thoracic"), lumbar, sacral, and coccygeal. There are typically 7 cervical vertebrae (C1-C7), 12 thoracic vertebrae (D1-D12), five lumbar vertebrae (L1-L5), five sacral vertebrae, and four coccygeal vertebrae. As is known, in adulthood, the sacral and coccygeal regions generally unite to form two bones, namely, the sacrum 1 and coccyx 2. The other vertebrae generally stay separate.
Each vertebra consists of two essential parts, namely, an anterior solid segment ("body") and a posterior segment ("arch"). The arch is formed of two pedicle and two laminae, which support seven processes, namely, four articular, two transverse and one spinous.
Referring to FIG. 2, a top elevation view of seventh cervical vertebra C7 is shown. As can be seen, laminae 3 are two broad plates of bone which complete a neural arch by fusing together. Laminae 3 enclose and define spinal foramen 5. Spinal foramen 5 serves for protection of the spinal cord. Spinous process 4 projects backward from the junction of laminae 3, and serves for attachment of muscles and ligaments. The distinctive character of seventh cervical vertebra C7 is the existence of a long and prominent spinous process 4. For this reason, seventh cervical vertebrae C7 is often referred to as "vertebrae prominent."
Referring to FIG. 3, a lateral elevation view of thoracic vertebra 30 is shown. As can be seen, each lamina 3 is broad, thick and imbricated. For thoracic vertebra 30, spinal foramen 5 is small and of a circular form (not shown). Spinous process 4 for thoracic vertebrae are long, triangular on transverse section, directed obliquely downward and terminate in a tubercular extremity.
Referring to FIG. 4, a top elevation view of lumbar vertebra 40 is shown. As can be seen, each lamina 3 is broad and short, and spinal foramen 5 is triangular. Spinous process 4 is thick and broad.
As can be seen from FIG. 1, spinous processes 4 is prominent, especially from seventh cervical vertebra C7 down to fifth lumbar vertebra L5. In referring to the "back", it should be understood that any and all thoracic and lumbar vertebrae are included, as well as, seventh cervical vertebra C7.
Because spinous process 4 has very little tissue between it and the exterior surface of skin of the back, its prominent extension often acts as a pile for driving into the back causing fracturing, compression or dislocation of vertebra. If a vertebra is displaced, it may pinch to sever the spinal cord.
Numerous types of injury may occur to the back. With regard to the present invention, there are several types of back injuries of which the mechanics and biomechanics will be examined.
The mechanisms of injury to the back may be thought of in three basic ways: compression, flexion-extension, and direct trauma.
Compression occurs when a load is applied in-line with the vertebral column. For example, as shown in FIG. 1, a force 16 applied in-line to spine 10 will cause compression injury. Injury to vertebrae of spine 10 depends on the relative position of the spine during impact. If spine 10 is bent in flexion, as shown in FIG. 23, the load is mostly carried by the vertebral body (e.g., body 17 as shown in FIGS. 2-4). As a result, a compression fracture 201 occurs as shown in FIG. 23. Because the vertebral ligaments remain in tact, damage to spinal cord 10 with resulting nerve injury is rare. An exception to this rarity occurs when the impact results in extrusion of the vertebral discs into the spinal canal. In that case, spinal cord compression occurs and nerve damage is typical.
If spine 10 is bent, as shown in FIG. 24, compressing load 16 is carried by facets 202 and spinous processes 4. In the case where the spine is bent and extended, disruption of the ligaments is more .likely to result. This produces an unstable situation leading to spinal cord 10 compression and resulting nerve damage. In addition, if vertebral body 17 is crushed, its posterior part is more likely to be affected causing fracture 203. Fragments from fracture 203 are likely to enter the spinal canal. These fragments often cause compression of the spinal cord resulting in nerve damage.
The second type of mechanism of injury to the back is due to flexion-extension. Flexion-extension injuries, as shown in FIG. 25, often are a result of automobile accidents. This is because the occupant is restrained by a seat belt. A traction force 18 applied along spinal column 10 causes posterior ligaments 204 to be stretched. Traction force 18 also may cause parts of the vertebral arch to fracture and separate 205, as shown in FIG. 26. Besides spinal cord damage due to stretching, the spinal cord may also be damaged due to misalignment of vertebrae 20, as shown in FIG. 27. Misalignment 206 causes fracturing of vertebrae, as well as, spinal cord damage.
Injury from extension alone, as shown in FIG. 28, are uncommon. The main reason is the anterior vertebral ligaments 210 are strong. The combined structure of facets 202 and spinous process 4 resists rotation. Thus, the combination of ligament strength and rotation resistance prevents many injuries. However, when extension is combined with compression or direct impact, devastating injury may result. Typically, the vertebral arch and ligaments fail, causing the spinal cord to be directly impacted. Bleeding in the spinal cord typically results. As the spinal cord is contained within the spinal foreman 5, swelling results in cord compression. The result in lack of blood flow due to swelling causes nerve damage.
The third mechanism of injury to the back is direct trauma. Forces applied directly and transverse to the vertebral column may have two important consequences: first intervertebral ligaments, as well as the entire vertebrae itself, may fail allowing lateral displacement and cord compression or shearing; and second, failure of the vertebral arch and spinal cord impact resulting in bleeding into the spinal cord causes swelling in a confined space resulting in interruption of blood flow to the nerves and resulting in damage.
Motorcyclists are typically most vulnerable to compression, extension and direct impact injuries. Flexion and traction injuries are not as often encountered, as with automobile occupants, because a motorcyclist's lower body is not restrained by a seat belt.
While compression forces are difficult to prevent without a rigid structure between the skull and pelvis, their impact may be diminished by preventing extension, so that the load is carried by the anterior part of the vertebral bodies.
Extension should also be limited or prevented because, if the spinal column is in an extended position, direct transverse forces are more likely to cause damage to the spinal cord.
The impact of direct transverse forces may be diminished by spreading the load over a larger area and by directing it away from the spinous process.
Referring to FIG. 5, a top broken out cross section view of the back is shown. Many anatomical details have been omitted in order to avoid confusion. As shown, vertebra 20 comprises two lamina 3, spinous process 4 and spinal foramen 5. Disposed in spinal foramen 5 is spinal cord 6. As shown, ribs 13 and muscles 12 are covered by skin 11.
A first biomechanism of injury to the back is shown in FIG. 6. The injury is caused by impact to the back, and in particular, spinous process 4. Because spinous process 4 protrudes so significantly, it is a likely target for receiving a force of impact to the back, as shown by reference as force 14. Moreover, because there is so little tissue protecting tip 8 of spinous process 4, there is little protection against direct impact to spinous process 4.
As shown in FIG. 6, impact to spinous process 4 often causes vertebra 20 to break at each lamina 3. It is common for lamina 3 to break as shown during severe impact to spinous process 4. This causes spinous process 4 to be pushed into spinal cord 6.
Because spinal cord 6 is contained within a solid bony compartment, spinal foramen 5, it is compressed when spinous process 4 is pushed into spinal cord 6. Thus, the blood supply to the nerves inside spinal cord 6 is typically cut-off. This results in further injury to the individual. Unfortunately, much of this type of injury is permanent. Not only may the blood supply be cut-off, tissue and nerve damage may occur due to the impact on spinal 6 cord from spinous process 4.
In a second biomechanism of injury to the back, laminae 3 does not break, but rather the arch breaks at locations 303, namely, where the arch attaches to body 17 of vertebra 20. In this type of injury, the spinal cord is often compressed, as the arch is pushed down against body 17 causing compression of vertebra 20.
A third biomechanism of injury to the back is shown in FIG. 7. As shown, force 15 is supplied laterally against the back and, in particular, spinous process 4. This causes each lamina 3 to break. The break and pressure exerted by force 15 causes spinous process 4 to be pushed against spinal cord 6. Again, as spinal cord 6 has no where to go due to its containment within spinal foramen 5, damage to nerves and tissue of spinal cord 6 may occur, as well as, cut-off of blood supply.
To protect the back against injury, devices have been devised. In fact, most groups that organize motorcycle races require back guards. Presently, two types of back guard are typically employed.
A first type of back guard 23 is shown in FIG. 8. Back guard 23 comprises pad 21 and attach thereto plastic sheet 22. Pad 21 is typically made of foam or cloth, or a combination of both. While back guard 23 does provide some protection, it is not contoured for the spine. Therefore, no relief is provided for spinous process 4. During impact to the back, forces are still directly transferred to spinous process 4, albeit through back guard 23. Thus, little of the force of impact is transmitted to surrounding tissue and anatomical structures. Damage to the surrounding tissue and anatomical structures is typically less severe.
Also, sheet 22 is made of a flexible material. This is to allow normal flexing of a wearer's spine. Flexible materials tend to distort and deform under loading. Thus, forces are not dispersed, as much as would be desired, away from spine 10.
Moreover, due to the flexible nature of plastic sheet 22, an impact to the side of the back may also be transmitted to vertebral process 4. Thus, for example, force 24 may be transmitted along plastic sheet 22 down and up against vertebral process 4. Had sheet 22 been omitted, transmission of force 24 to vertebral process 4 may not have occurred. Rather, force 24 would be direct against muscle tissue 12 and ribs 13, which damage thereto is typically less severe. Thus, back guard 23 may cause injury, which may not have occurred with omission of back guard 23.
A second type of back guard is shown in FIGS. 9-11.
In FIG. 9, back guard 25 comprises plastic sheet 22 and pad 21. However, plastic sheet 22 of back guard 25 is contoured to provide cavity 27. Cavity 27 has a maximum height of approximately 1 cm and an opening width of approximately 8 cm.
A top elevation view of back guard 25 is shown in FIG. 10. As shown, back guard 25 comprises straps 26 attached to pad 21. Also attached to pad 21 are a plurality of plastic sheets 22. Plastic sheets 22 are typically attached to pad 21 by rivets. Plastic sheets 22 are over laid as to form a series for protecting the back. As shown by dash line 31, the tail portion 32 (shown in FIG. 11) of plastic sheet 22 fits in cavity 27 as provided by contour 33, as shown in FIG. 11.
FIG. 11 shows the underside of plastic sheet 22, absent rivets. As shown, plastic sheet 22 has contour 33 for providing cavity 27. Additionally, tail 32 is used for under laying with another plastic sheet 22.
Unfortunately, because tail 32 is located in cavity 27, the amount of relief for spinous process 4 is reduced. At some locations in cavity 27, no relief is provided due to over laying of plastic sheets 22. In fact, at some locations within cavity 27 where plastic sheets have been over laid, no relief in either the vertical or horizontal direction is provided for spinous process 4. Thus, the same problems associated with back guard 23 are also present with back guard 25. Moreover, any amount of relief provided by cavity 27 is insufficient to protect spinous process 4 from severe impact.
Another problem with back guard 25 is illustrated in FIG. 12. In FIG. 12, if spine 10 is bent in a backward direction 41, then back guard 25 will also flex in backward direction 41. Because back guard 25 is not interlocked, but over laid, there is no mechanism to prevent tail 32 from articulating into spine 10 when being flexed in backward direction 41, as shown with reference to a portion of back guard 25 in FIG. 12. Tail 32 may be pushed up against spinous process 4 causing some of the injuries described above. Thus, the force of impact may be directed to a small area on spine 10, namely, focusing force to edge 34 and against spine 10. Moreover, force from tail 32 against spinous process 4 may cause: spinous process 4 to break-off from vertebra 20, vertebral displacement of vertebra 20, or fracture of vertebra 20. An injury which may not have occurred with the omission of back guard 25. Additionally, because plastic sheets 22 of back guard 25 are not interlocked, there is no mechanism for preventing hyperextension of spine 10.